This invention relates to systems and methods of film deposition, and more particularly, to improved systems and methods for depositing a high quality film onto a large area substrate.
In recent years, liquid crystal cells have been developed to form high quality displays that are light-weight and consume low power. These liquid crystal cells typically include two glass substrates with a layer of a liquid crystal material sandwiched therebetween. Electrically conductive films are patterned on the substrates to form circuit elements such as thin film transistors (TFTs). The substrate can be connected to a power source to change the orientation of the liquid crystal material such that various regions of the liquid crystal display can be selectively energized using the TFTs.
Reactors for depositing films onto the large area glass substrates typically deploy plasma enhanced chemical vapor deposition (PECVD) equipment. A high frequency power supply is typically used to induce a breakdown of process gases within the PECVD process chamber. As glass substrates are typically much larger than silicon substrates, the dimensions of the electrode may approach the quarter-wavelength of the power supply frequency. Such conditions lead to an uneven discharge of electrical energy over the surface of the large substrate. This non-uniform voltage distribution can result in an uneven film deposition on the substrate surface.
Traditional solutions to the uneven film deposition have involved adjusting various process variables, including pressure, gas composition, flow rate, radio frequency (RF) power level, and electrode spacing, among others. Adjusting these process variables works well for relatively small substrates. However, as the size of the substrate, the size of the chamber and the frequency of the power supply increase, the film deposited on the substrate may be non-uniform.
In general, in one aspect, an apparatus is disclosed for distributing RF power outputs to a first electrode in a parallel plate electrode system for generating plasma in depositing films on a substrate. The apparatus includes a RF power supply and a matching network coupling the RF power supply to multiple points distributed on the first electrode.
In some implementations, the RF outputs are coupled directly to a backing plate which serves as the electrical connection between outputs from one or more matching networks and a shower head.
In one exemplary implementation, the apparatus includes a load capacitor for receiving a radio frequency power input and an inductor having first and second ends, with the first end coupled to the load capacitor. The apparatus also includes multiple drive capacitors each of which electrically couples the second end of the inductor to a different one of multiple points distributed on the first electrode.
In some implementations, the capacitance of each drive capacitor is user-selectable. Similarly, in some implementations, the points on the first electrode to which the drive capacitors are coupled are user-selectable. Such features allow a user to adjust the values of the drive capacitors as well as the locations of the points on the first electrode to which the drive capacitors are coupled to improve the uniformity of the deposited film.
The apparatus can be incorporated into a system for depositing a thin film, where the system also includes, for example, a vacuum chamber in which a substrate to be processed and the first electrode are positioned, a process gas source coupled to the first electrode to introduce a gas stream into the chamber, and a heater for heating the substrate in the chamber.
The distributed impedance matching network can be used in various plasma enhanced processing systems which include one or more RF power supplies. In systems having multiple power supplies, the distributed matching network can be used to couple, for example, a high frequency power supply to the first electrode.
In another aspect, a method of processing a thin film on a substrate includes providing a radio frequency power input to a load capacitor and an inductor where the inductor is coupled to multiple drive capacitors, and applying an output of each respective drive capacitor to a different one of multiple points distributed on an electrode. In some implementations, the capacitance of each drive capacitor can be adjusted to arrive at a composite predetermined value. As previously noted, the values of the drive capacitors as well as the locations of the points on the electrode to which the drive capacitors are coupled can be adjusted to improve the uniformity of the deposited film.
Various implementations include one or more of the following advantages. By supporting movable tie points from the RF power supply outputs to the electrode, the system provides a spatial control variable which allows a user to select or adjust the locations on the electrode to which the drive capacitors are coupled electrically. Additionally, by allowing a user to select the individual values of the capacitors, the system provides an electrical control variable. The spatial control variable and the electrical control variable supplement the traditional process variables, including pressure, gas composition, flow rate, RF power level, and electrode spacing such that more uniform films are deposited. Substantially uniform films interface better to subsequently deposited layers. Other film properties such as density and stress also are improved, and a high deposition rate can be achieved.
Other features and advantages of the invention will become apparent from the following description, including the drawings and claims.